The invention relates to methods of fabricating ceramic materials and also to products that can be obtained by performing such methods.
Chemical vapor infiltration (CVI) methods are known for densifying fiber preforms. Methods of that type consist in infiltrating a gas mixture containing all of the elements for forming the material within a porous preform in order to densify the preform. By way of example, that type of method is described in application FR 2 784 695. Chemical vapor infiltration is derived from the technique of chemical vapor deposition (CVD) and it possesses a deposition rate that is constant over time. It is a method that gives good properties to the material. Nevertheless, in order to obtain a ceramic matrix composite (CMC) that is uniform, while avoiding premature clogging at the periphery of the preform, it can be necessary to operate at low pressure and at relatively low temperature (≤1100° C.) in order to slow down growth rates. This leads to fabrication durations for CMC parts that are long and makes the method expensive. Machining may be needed in order to reopen the pores at the periphery so as to give the gases access to the core. Nevertheless, matrix densification may be stopped once the porosity has reached a value close to 10% to 15%, because of the presence of macropores.
Also known is the slurry or ceramic or sol-gel technique, which consists in impregnating fiber preforms with a slurry or a sol (a mixture of ceramic particles of sub-micrometer dimensions, sintering additives, and liquid solvents) followed by drying and sintering the whole at 1600° C. to 1800° C. under pressure. By way of example, such a method is described in EP 0 675 091 and in the publication by J. Magnant, L. Maillé, R. Pailler, J-C. Ichard, A. Guette, F. Rebillat, and E. Philippe entitled “Carbon fiber/reaction-bonded carbide matrix for composite materials—Manufacture and characterization”, published in J. Europ. Ceram. Soc. 32 (16) 2012, pp. 4497-4505. Nevertheless, preparing carbon xerogels can involve using substances classified as CMR (carcinogenic, mutagenic, or toxic for reproduction), which can make industrial production difficult.
The various known techniques may either be used independently, or else they may be combined with one another in order to form hybrid methods. Various examples of hybrid methods are described below.
Slurry and CVI hybrid methods are known that combine the slurry technique (without sintering additives) with the gaseous technique. After impregnating the fiber preform with the slurry, the matrix can subsequently be densified by conventional CVI using green composite. Nevertheless, the great compactness of an agglomerated (sub)micrometer powder constitutes a brake to good infiltration. The core of the material densifies poorly because of the premature closure of the pores at the periphery of the material. Reactive species find it difficult to penetrate into the small pores and their concentration drops off very quickly going from the periphery to the core, thereby greatly slowing down and then preventing growth of the consolidation layer. Tang et al. (S. F. Tang, J. Y. Deng, S. J. Wang, W. C. Liu, K. Yang “Ablation behaviors of ultra-high temperature ceramic composites” Materials Science and Engineering A 465 (2007) pp. 1-7) have nevertheless made composites from green compacts of micrometer powders of ZrB2, SiC, HfC, and TaC consolidated by pyrolytic carbon CVI. Under such circumstances, the continuous matrix phase is made of pyrolytic carbon. By replacing conventional CVI with pulsed CVI, it is possible to consolidate micrometer powders (4 micrometers (μm) to 5 μm) forming a green compact of millimeter thickness (N. K. Sugiyama and Y. Ohsawa “Consolidation of Si3N4 powder-preform by infiltration of BN using the pulse CVI process” Journal of Materials and Science Letters 7 (1988) pp. 1221-1224). Purging and filling the green compacts makes it possible periodically to reduce the natural concentration gradient of the gaseous species between the core and the periphery. However feasibility has not been reported for sub-micrometer powders, and the method appears to be difficult to industrialize.
A pre-ceramic slurry and resin hybrid method makes it possible to prepare a matrix from an impregnated powder and a pre-ceramic resin (Peter Greil, Net shape manufacturing of polymer derived ceramics, J. Europ. Ceram. Soc. 18 1998, pp. 1905-1914). The increase in the volume of the powder makes it possible to compensate in part for the shrinkage in volume of the resin during pyrolysis.
Work has recently been carried out (Matrices nanostructurées élaborées par voie liquide: application aux composites à matrice céramique [Nanostructured matrices prepared by a liquid technique: application to ceramic matrix composites], Thesis 4323 Université Bordeaux 1, 2011, and L. Maillé, M. A. Dourges, S. Le Ber, P. Weisbecker, F. Teyssandier, Y. Le Petitcorps, R. Pailler, “Study of the nitridation process of TiSi2 powder”, Applied Surface Science 260 (2012), pp. 29-31) for preparing a matrix by volume expansion by causing a powder that is impregnated within the preform to react with a gas. The system that has been studied until now is nitriding a TiSi2 powder with dinitrogen at normal pressure, leading to one of the greatest volume increases, of the order of 60%. In that work, the objective was to prepare a low cost matrix by using Nicalon® fibers that are unstable above 1100° C., and during nitriding a treatment temperature was used that was less than or equal to 1100° C. That work has shown that under such conditions, nitriding of the powder is relatively slow and incomplete. The problem relates to nitriding silicon because of a slow conversion rate.
There therefore exists a need to have novel methods of fabricating ceramic materials at low cost that are suitable for use on an industrial scale and in which it is possible to make use of a treatment temperature that is relatively low.
In particular, there is a need to have novel methods of densifying fiber preforms that can be used on an industrial scale and that present a cost and a working temperature that are relatively low.
There also exists a need to have novel methods of fabricating ceramic materials at low cost in which the chemical reaction used is complete.
There also exists a need to have novel methods of fabricating ceramic materials in which the materials obtained are substantially free of residual free silicon.
There also exists a need to have novel ceramic materials that present mechanical properties that are very satisfactory and a microstructure that is uniform.